1. Field of the Invention
The present invention is directed to methacrylic polymer compositions which may be used to form plastic products which have increased resistance to sterilizing radiation. In particular, this invention relates to objects formed from poly(methyl methacrylate) which show maintenance of transparency and lack of yellowing on exposure to sterilizing radiation, such as gamma or electron beam irradiation, for sterilization purposes.
2. Description of the Prior Art
Poly(methyl methacrylate), including clear, impact modified forms, is a useful polymer for forming articles in the medical field, such as syringes, spikes, connectors and luers, suction devices, urine meters, blood plasma separators, drip chambers, cuvettes, dialyzer casings, chest drainage units, bottles for fluids, vaginal speculums, flow valves, aspirators, pump housings, containers for operating instruments, and the like, as the polymers are crystal clear, easily molded into the desired form, compatible with poly(vinyl chloride) (PVC) tubing, bondable to other plastics, resistant to deformation under warm conditions, reasonably tough and readily processed into useful objects. Such objects further have dimensional stability, have design flexibility, and can readily be disposed of after one-time use by incineration.
For many uses in the medical field, sterilization of these articles is required. Sterilization is often accomplished by exposure of the article to low level gamma or electron beam radiation (which will be designated "sterilizing radiation"). This sterilization is usually done for "use-once" or throw-away articles, although the invention would also apply to repeated sterilization.
The amount of radiation utilized to accomplish sterilization is well above "background" radiation, and when objects of polymerized methyl methacrylate are exposed to such radiation, the polymer is subject to yellowing. This yellowing reduces the light transmission of the poly(methyl methacrylate) and alters its appearance in an esthetically unfavorable way. Much of the yellow color will be lost on aging, especially if the sterilized sample is maintained at an elevated temperature, such as about 60.degree. C., but the level of residual color is still unattractive when compared to the non-irradiated sample.
Poly(methyl methacrylate), although far less prone to discolor and lose properties on exposure to sunlight than other plastics, may be further stabilized against discoloration and loss of properties by use of any of a variety of ultraviolet stabilizers, such as benzotriazoles, hydroxyphenylbenzotriazoles, hindered amines, such as derivatives of 2,2,6,6-tetramethylpiperidine, salicylate esters, such as phenyl salicylate, o-hydroxybenzophenones, and the like. However, these materials are ineffective against discoloration by sterilizing radiation and certain ones may contribute further to the discoloration.
The art has also attempted to improve gamma radiation resistance of poly(methyl methacrylate) by addition of propyl alcohol. An article by A. S. Belichenkol et al, entitled "On the Mechanism of Polymer Destruction under UV and Gamma Irradiation: The Influence of Low Molecular Weight Additives Related to Vibrational Cross-Relaxation", Sixth Symposium on Radiation Chemistry, (1986), pages 535-538, discloses that polymer samples prepared by bulk radical polymerization of monomer compositions of methyl methacrylate which incorporated from 5 to 20% n-propyl alcohol showed improved resistance to gamma radiation, as judged by much slower development of UV-adsorbing chromophores. The large amounts of at least 5% propyl alcohol, however, would be expected to degrade the physical and mechanical properties of these polymers. This publication presents no comparative data between stabilized and non-stabilized polymers, but contrasts only irradiated and non-irradiated polymers of the same alcohol content in a qualitative statement.
Polymers of methyl methacrylate containing levels of alcohols less than 0.5%, insufficient effectively to inhibit yellowing on exposure to sterilizing radiation, have been known for some years. Recently, polymers of methyl methacrylate containing similar amounts of similar alcohols as described in the present invention have been found to be effective in resistance to ultraviolet degradation; such compositions are disclosed in U.S. patent application Ser. No. 385,139, incorporated herein by reference. It is well-known that commercial ultraviolet stabilizers in poly(methyl methacrylate) may protect against ultra-violet degradation but are ineffective or even produce higher color on exposure to sterilizing radiation, such as gamma irradiation. Thus, there is no reason to anticipate similar stabilization behavior for alcohol-containing poly(methyl methacrylate) on exposure to differing types of radiation of differing wave lengths and energy.
Thus, it would be desirable if poly(methyl methacrylate) could be provided which did not discolor on exposure to sterilizing radiation, while maintaining or only slightly decreasing its other desirable physical properties, and it is an object of the present invention to provide such poly(methyl methacrylate).